Control voltage limiter for cathode ray tube receivers



April 29, 1941 w. DILLENBURGER ETAL 2, 8

CONTROL VOLTAGE LIMITER FOR CATHODE RAY TUBE RECEIVERS Filed Feb. 16,19.38

' I I I 1%? aw W Patented Apr. 29, 1941 accuse CONTROL VOLTAGE LIMITER.FOR CATHODE RAY RECEIVERS Wolfgang Dillenburger, Berlin-Zehlendorf, andKurt Thiim, Berlin-Lichterfelde-Sud, Germany, assignors to the firm ofFernseh Aktiengesellschaft, zehlendorf, near Berlin, Germany ApplicationFebruary. 16, 1938, Serial No. 190,884 In Germany February 17, 1937 5Claims.

which the image is divided into two or more partial images. In thiscase, one partial image will not have exactly thesame brllliancy as theother, and if one partial image already shows strong radiation at thebright spots, the other image will, at the same spots, still be belowthe limit of strong light radiation. In this case a particularlyannoying flickering appears.

If the characteristic of such a cathode ray tube is measured, and showsthe light intensity H as a function of the control voltage St, a curveof the type indicated by a. in Fig. 1 is obtained. Curve a first risesgradually and then very rapidly, that is, a slight increase of thecontrol voltage beyond a certain value of the curve'produces a suddenincrease of the maximum light intensity. This peculiarity of thecharacteristic led to the operation in the first region of thecharacteristic, so that even maximum amplitudes of picture signal stilllie below the point A inv order to prevent too strong a radiation of thebright spots. The contrast of the image is thereby also reduced to acomparatively small range of light intensity. In order to obtain acharacteristic which is approximately linear, a

circuit according to the invention is used by which the limitation ofthe control voltage to values is obtained, which values remain below thepoint at which excess radiation occurs. The final result is that anexactly or at least approximately linear light intensity characteristicis obtained.

In order to accomplish this, it is possible, for instance, to operatethe last amplifier stage in the range of the lower knee, and in thismanner to suppress all values of the control voltage exceeding a valuedetermined by the properties of the cathode ray tube. Accordingly, thestraightening out of the characteristic is dependent upon the tubecharacteristic of the final stage and is variable only within smalllimits by suitable choice of the operating point. Furthermore, theadjustment is very critical because the variation of the operating pointsimultaneously influences several properties of the receiver so that,for instance, the synchronization must be readjusted and similarrearrangements made.

An arrangement which allows a change and any desired adjustment of thecharacteristic without making further adjustments of the receiver,consists in the application of a circuit which switches one or severalresistors in parallel with the control-grid resistor if the controlvoltage exceeds a certain predetermined value. Such a circuit maycontain, for instance, one or several diode tubes, as shown in Fig. 2.Parallel to the plate resistor I of a video amplifier are two diodetubes 2 and 3, which are held at different bias and each of which isconnected in series with resistors 4 and 5, respectively. As soon as thevoltage produced across the resistor I is less than the rectifier bias,an additional current flows through the diode tubes, the magnitude ofwhich is substantially determined by the resistors 4 and 5. Thus, anadditional load is put on to the preceding amplifier stage so that thevoltage at the upper end of resistor l increases more slowly because ofthe parallel connection of the resistors 4 and 5, if the picture signalvoltage exceeds a certain value. The characteristic of such a circuit,which shows the output voltage Sta as a function of the input voltageSt, is shown by the curve I; of Fig. 1. This characteristic has twoknees which correspond to the bias values of the diode tubes. The slopeof the curve is predetermined by the value of the resistors 4 and 5.

The bias voltages and the resistors are adjusted to such values,according to the invention, that the voltage at the Wehnelt cylinder ofthe cathode ray tube does not exceed the value at which excess radiationoccurs, even for the greatest input voltage. The device is adjusted forgood gradation for the dark spots of the image, and by changing the biasand the resistance in the diode circuits the characteristic isinfluenced in such a manner that it possesses the characteristic b. Withsuch an adjustment, the light intensity becomes a substantially linearfunction of the input voltage.

The circuit can also be made to operate with only one diode tube, orwith other circuit components operating in a corresponding manner.

Having thus described our invention, we claim:

1. A signal translating system for a cathode ray image receiver whereinthe luminescence effect of the cathode ray on a screen is a nonlinearfunction of the signal potentials impressed on a control electrode ofsaid receiver, comprising an amplifier having an output circuit inlinear luminescence characteristic of said cathode ray image receiver.

2. A signal translating system for a cathode ray image receiver whereinthe luminescence effect of the cathode ray on a screen is a non-linearfunction of the signal potentials impressed on a control electrode ofsaid receiver, comprising an amplifier having an output circuitincluding a reing means for deriving across a resistor in an amplifieroutput circuit signals for application to said control electrode, andmeans connected in circuit with said resistor and responsive to saidsignals for automatically changing the effective value ofsaid resistorinversely in accordance with the am litude of said signals.

4. A signal translating system for a cathode ray image receiver whereinthe intensity of the cathode ray is a function of the potential appliedto a control electrode of said receiver, comprising means for derivingacross a resistor in an amplifier output circuit signals for applicationto said control electrode, and means connected in circuit with saidresistor and responsive to said signals for automatically changing theeffective sistor from which signals are derived for application to acontrol electrode of said cathode ray image receiver, and a dioderectifier and resistor in series and shunting said first-mentionedresistor, said diode being biased negatively to a value rendering saiddiode incapable of passing current until the signal voltage across saidfirst mentioned resistor reaches a predetermined value, whereby saiddiode will become conductive at and above said predetermined value ofsignal to reduce the effectiveness of said first-mentioned resistor.

3. A signal translating system for a cathode ray image receiver whereinthe intensity of the cathode ray is a function of the potential appliedto a control electrode of said receiver, comprisvalue of said resistorin steps inversely in accordance with the amplitude of said signals.

5. A'signal translating system for a cathode ray image receiver whereinthe intensity of the cathode ray is a function of the potential appliedto a control electrode of said receiver, comprising means for derivingacross a resistor in an amplioutput circuit signals for application tosaid control electrode, and means comprising a plurality of networks,each including a series-connected diode rectifier, a source of potentialand a resistor, in shunt with said first-mentioned resistor forautomatically changing the effective value of said first-mentionedresistor inversely in accordance with the, amplitude of said signals.

WOLFGANG DILLENBURGER. KURT THoM.

